The invention relates to a method of generating extremely short-wave radiation, in which a medium is transported through a vacuum space and each time a part of the medium in the vacuum space is irradiated with a pulsed and focused energy-rich laser beam, said part of the medium being converted into a plasma emitting extremely short-wave radiation.
The invention also relates to a method of manufacturing a device by means of this radiation. Furthermore, the invention relates to an extremely short-wave radiation source unit and to a lithographic projection apparatus provided with such a radiation source unit.
Extremely short-wave radiation is understood to mean extreme UV (EUV) radiation, which can be used in lithographic projection apparatuses, and X-ray radiation for various applications.
Said medium may be a mobile medium, i.e. a medium which does not have a solid shape but whose shape is determined by the holder accommodating the medium or the guide by which the medium is transported. However, the medium may be alternatively a solid medium such as a metal which can be locally exploded by laser beam bombardment, in which the released particles form a plasma emitting extremely short-wave radiation. The metal medium may be a tape or a wire transported through the vacuum, or source space.
A lithographic apparatus is used, inter alia, in the manufacture of integrated electronic circuits or ICs for imaging an IC mask pattern, present in a mask, each time on a different IC area of a substrate. This substrate, which is coated with a radiation-sensitive layer, provides space for a large number of IC areas. The lithographic apparatus may also be used in the manufacture of, for example, liquid crystalline image display panels, integrated, or planar optical systems, charge-coupled detectors (CCDs) or magnetic heads.
Since an increasingly large number of electronic components is to be accommodated in an IC, increasingly smaller details, or line widths, of IC patterns must be imaged. Consequently, increasingly stricter requirements are imposed on the imaging quality and the resolving power of the projection system in the apparatus, which projection system is generally a lens system in the current lithographic apparatuses. The resolving power, which is a measure of the smallest detail which can still be imaged, is proportional to xcex/NA, in which xcex is the wavelength of the imaging, or projection, beam and NA is the numerical aperture of the projection system. To increase the resolving power, the numerical aperture may, in principle, be enlarged and/or the wavelength may be reduced. An increase of the currently already fairly large numerical aperture is no longer very well possible in practice because the depth of focus of the projection system, which is proportional to xcex/NA2, will become too small and the correction for the required image field will be too difficult.
The requirements to be imposed on the projection system may be mitigated, or the resolving power may be increased while these requirements are maintained if a step-and-scan lithographic apparatus is used instead of a stepping lithographic apparatus. In a stepping apparatus, a full-field illumination is used, i.e. the entire mask pattern is illuminated in one run and imaged as one whole on an IC area of the substrate. After a first IC area has been illuminated, a step is made to a subsequent IC area, i.e. the substrate holder is moved in such a way that the next IC area is positioned under the mask pattern, whereafter this area is illuminated, and so forth, until all IC areas of the substrate are provided with a mask pattern. In a step-and-scan apparatus, only a rectangular or annular segment-shaped area of the mask pattern and hence a corresponding sub-area of a substrate IC is illuminated, and the mask pattern and the substrate are synchronously moved through the illumination beam, while taking the magnification of the projection system into account. A subsequent area of the mask pattern is then each time imaged on a corresponding sub-area of the relevant IC area of the substrate. After the entire mask pattern has been imaged on an IC area in this way, the substrate holder performs a step, i.e. the start of a subsequent IC area is introduced into the projection beam and the mask is set, for example, to its initial position whereafter said subsequent IC area is scan-illuminated via the mask pattern.
If even smaller details are to be satisfactorily imaged with a step-and-scan lithographic apparatus, the only possibility is to reduce the wavelength of the projection beam. In the current step-and-scan apparatuses, deep UV (DUV) radiation is already used, i.e. radiation having a wavelength of the order of several hundred nanometers, for example, 248 nm or 193 nm from, for example, an excimer laser. Another possibility is the use of extreme UV (EUV) radiation, also referred to as soft X-ray radiation, with a wavelength in the area of several nm to several tens of nm. Extremely small details, of the order of 0.1 xcexcm or smaller, can be satisfactorily imaged with such a radiation.
Since there is no suitable lens material available for EUV radiation, a mirror projection system must be used for imaging the mask pattern of the substrate, instead of a hitherto conventional lens projection system. For forming a suitable illumination beam of the radiation from the EUV radiation source, mirrors are also used in the illumination system. The article xe2x80x9cFront-end design issues in soft-X-ray lithographyxe2x80x9d in Applied Optics, Vol. 23, No. 34, 01-12-93, pp. 7050-56 describes a lithographic apparatus in which EUV radiation is used and whose illumination system comprises three mirrors and the imaging, or projection, system comprises four mirrors. As is described in the article xe2x80x9cDebris-free soft X-ray generation using a liquid droplet laser plasma targetxe2x80x9d in: Applications of laser plasma radiation II, SPIE 2523 , 1995, pp. 88-93, EUV radiation can be generated by focusing a laser beam on water droplets. The required stable flux of individual micro water droplets can be obtained by means of a capillary glass tube which is vibrated by a piezoelectric driver. Due to the high temperature, each water droplet impinged upon by the laser beam is consecutively converted into a plasma which emits EUV radiation.
In EUV lithographic apparatuses, it is a great problem to illuminate the substrate at a sufficiently high intensity. A first cause of this problem, which applies to all EUV apparatuses, is that the mirrors used are considerably less than 100% reflecting. Each of these mirrors has a multilayer structure whose composition is adapted as satisfactorily as possible to the wavelength of the projection beam used. Examples of such multilayer structures are described in U.S. Pat. No. 5,153,898. A multilayer structure which is frequently mentioned in literature is the structure consisting of silicon layers alternating with molybdenum layers. For radiation coming from a plasma source, these layers theoretically have a reflection of the order of 73% to 75%, but in practice the reflection is currently not larger than 65%. When said number of seven mirrors is used, each with a reflection of 68%, only 6.7% of the radiation emitted by the source reaches the substrate. For a lithographic apparatus, this means in practice that the illumination time should be relatively long in order to obtain the desired quantity of radiation energy on a substrate, and that the scanning velocity would be relatively small, particularly for a scanning apparatus. However, it is essential for these apparatuses that the scanning velocity is as high as possible and the illumination time is as short as possible so that the throughput, i.e. the number of substrates which can be illuminated per unit of time, is as high as possible. This can only be achieved with an EUV radiation source which supplies sufficient intensity. A second cause of the problem relates to the fact that the generated EUV radiation may be absorbed as little as possible, which means that the product of the path length traversed by the EUV radiation and the pressure in the space in which this radiation propagates must not exceed a given value. This space must be a vacuum space with a pressure of, for example, the order of 0.1 mbar if said path length is of the order of 1 m. For a larger path length, the vacuum requirement is also stricter, for example, the pressure in the vacuum space must not be higher than 10xe2x88x923 mbar. In a water plasma source, there is the problem that the vapor pressure of water at room temperature is approximately 23 mbar, which does not comply with the vacuum requirement which must also be maintained in the holder of the radiation source. Moreover, the water vapor may get through the apertures provided in the holder for passing the laser beam and leave this holder so that it will deposit on the mirrors of the illumination system and the projection system and attack these mirrors, thus reducing their reflection. Similar problems occur when other liquids and gases are used as starting media for generating EUV radiation. Such a gas is, for example, xenon in the form of clusters. As regards physical state, such xenon clusters occupy a position between molecules and a solid material. When a cluster is irradiated with a high-power laser beam, the cluster emits EUV radiation. Inter alia, also energy-rich ions are then released, which may cause great damage. Also when a metal tape or wire is bombarded with a laser beam so as to generate EUV radiation, particles are released which may cause damage and may absorb the EUV radiation.
It is an object of the present invention to provide a method in which the above-mentioned problems in EUV sources, and similar problems in X-ray sources, can be obviated. To this end, this method is characterized in that the medium is embedded in at least a viscous flow of rare gas which is transported through the vacuum space parallel to the direction of movement of the medium.
Vapor deposited from the mobile medium, excess medium which has not been converted into plasma and particles repelled by the plasma formed are taken along by the flow of rare gas and transported to the vacuum pump for the source space. It is thus achieved that the vacuum, or source, space remains sufficiently transparent to the extremely shortwave radiation, and it is sufficiently prevented that said elements of the medium can penetrate other spaces of the apparatus in which the mirrors of the illumination system and those of the projection system are situated. The flow of rare gas is a viscous flow so that it exerts a sufficiently large suction on the matter to be removed. The flow is preferably a laminar flow so that a return flow of the rare gas, and the elements of the medium present therein, is suppressed very effectively.
The method is preferably further characterized in that two viscous flows of rare gas are passed through a part of the vacuum space in which a part of the medium, which is not yet irradiated, propagates.
By making use of two or more extra flows of rare gas, the velocity profile of the rare gas, which profile may be disturbed by an interaction of the moving medium with the rare gas, can be restored.
The method is preferably further characterized in that helium is used as a rare gas.
The envisaged object can be eminently realized with helium which is the lightest of the rare gases and absorbs little extremely short-wave radiation. Instead of helium, for example, also argon may be used, which, on the one hand, can better drain the medium elements but, on the other hand, is more absorbing than helium.
It is to be noted that the article xe2x80x9cCharacterization and control of laser plasma flux parameters for soft X-ray lithographyxe2x80x9d in Applied Optics, Vol. 32, No. 4, 01-12-93, pp. 6910-6930 describes an EUV radiation source based on a metal medium and deals with a problem that may be caused by particles coming from the metal. It is proposed to fill the source space with helium at such a pressure that, on the one hand, collisions prevent the source particles from moving away from the source and, on the other hand, a minimal quantity of EUV radiation is absorbed by the helium. To this end, the helium must fill the entire source space so that more helium is required and the risk of absorption of EUV radiation is greater than in the radiation source unit according to the invention. The specific problems, which occur when liquid or gaseous media are used, are not mentioned in the article.
Furthermore, it is noted in the article xe2x80x9cDebris-free soft X-ray generation using a liquid droplet laser plasma targetxe2x80x9d in SPIE, Vol. 2523, Applications of laser plasma radiation II, 1995, pp. 88-93 that the number of contaminated particles formed during irradiation of ethanol or water droplets for generating EUV radiation, is three times smaller than the number of particles for irradiating a solid material. Then, however, source particles may still reach optical elements of the apparatus in which the EUV radiation is used. To avoid this, the article proposes use of a localized flow of rare gas. However, this flow is passed along the surface of the optical elements to be protected and is not passed through the source space. However, a larger quantity of helium is required for this purpose, and the absorption is greater again.
A first embodiment of the method is characterized in that a metal is used as a medium which, upon irradiation with a laser beam, forms a plasma emitting extremely short-wave radiation.
For the metal medium, various metals such as iron, tin and carbon are suitable. This medium preferably has the shape of a tape or a wire. A second embodiment of the method is characterized in that a liquid is used as a medium which, upon irradiation with a laser beam, forms a plasma emitting extremely short-wave radiation.
This embodiment is preferably further characterized in that a continuous flow of individual water droplets is used as the flow of liquid medium.
Water is a relatively clean medium and good results have already been achieved with a radiation source in which a flow of water droplets is used.
A third embodiment of the method is characterized in that a clustered gas is used as a medium which, upon irradiation with a laser beam, forms a plasma emitting extremely short-wave radiation.
This embodiment is preferably further characterized in that the gas is xenon.
Good results appear to be achieved with an extremely short-wave radiation source in which this gas is used.
The invention also relates to a method of manufacturing a device, in which the dimensions of the smallest details are smaller than 0.25 xcexcm, on a substrate, in which method different layers of the device are formed in successive steps by imaging by means of EUV radiation, for each layer, first a specific mask pattern on the substrate coated with a radiation-sensitive layer and by subsequently removing material from, or adding material to, areas marked by the mask image. This method is characterized in that the EUV radiation is generated by means of the method described hereinbefore.
The invention further relates to an extremely short-wave radiation source unit comprising:
a source space connected on a first side to a vacuum pump;
an inlet device on a second side of the source space for introducing the medium into the source space;
a pulsed high-power laser, and
an optical system for focusing the laser beam supplied by the laser on a fixed position within the source space where it passes. This radiation source unit is characterized in that the source space is connected on the second side to a rare gas inlet for establishing a viscous flow of rare gas in the source space enveloping the medium, which flow is parallel to the direction of movement of the medium.
A first embodiment of this radiation source unit, in which the source space is enclosed by a wall having apertures for causing the laser beam to enter into and exit from the source space and for causing the generated extremely short-wave radiation to exit from the source space, is characterized in that a tube is arranged in the source space on the second side of the source space and parallel to the direction of movement of the medium, which tube is connected to said inlet for establishing the viscous flow of rare gas.
This embodiment is preferably further characterized in that a second tube is arranged parallel to the first tube in the source space, which second tube is connected to said inlet for establishing a second viscous flow of rare gas parallel to the direction of movement of the medium.
A second embodiment of the radiation source unit is characterized in that the source space is formed by a first closed part on the first side, a second closed part on the second side and a central part which communicates with the ambience, in that the wall of the second source space part is formed by a tube which is connected to said rare gas inlet, and in that the wall of the tube and the wall of the first source space part have such a shape at the area of the central part of the source space that they constitute an ejector geometry.
Due to the extra suction caused by the ejector, or jet pump, geometry, it is prevented that rare gas can leak through the apertures and raise the pressure in the apparatus spaces accommodating optical components.
A third embodiment of the radiation source unit is characterized in that the source space is formed by a first closed part on the first side, a second closed part on the second side and a central part which communicates with the ambience, in that the wall of the second source space part is formed by an annular tube which is connected to said rare gas inlet, and in that the wall of the tube and the wall of the first source space part have such a shape at the area of the central part of the source space that they constitute an annular ejector geometry.
As compared with the second embodiment, this embodiment has the advantage that the tube which is a part of the jet pump is narrow so that a satisfactory operation of this pump is ensured, and that it also provides the possibility to create some distance between the focus of the laser beam and the position where the medium passes. Especially for the case where liquid droplets or gas clusters are used as a medium, the risk that a droplet or cluster is not impinged upon by the laser beam will be smaller. Since the density of the laser energy in this droplet or cluster then remains limited, the number of energy-rich ions and radicals repelled by the plasma may also remain limited.
A metal, a gas or a liquid may be used as a mobile, plasma-forming medium in the above-mentioned radiation source unit, as defined in claims 15-21.
Finally, the invention relates to a lithographic projection apparatus for imaging a mask pattern on a substrate provided with a radiation-sensitive layer, which apparatus comprises an illumination system for illuminating the mask pattern and a projection system for imaging the illuminated mask pattern on the substrate, the illumination system comprising an EUV radiation source, while the optical components of the illumination system and those of the projection system are present in a vacuum space. This apparatus is characterized in that the EUV radiation source is an EUV radiation source unit as described hereinbefore.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.